The invention relates generally to the field of image processing, and more particularly, to adjusting a method for adjusting the tone scale of a digital image.
Existing methods for modifying the tone scale and color characteristics of a digital color image include applying a tone scale function to each of the color channels independently, applying the tone scale function to the neutral channel and preserving the original pixel color difference signal, or applying the tone scale function to the low frequency image. In addition, a 3 dimensional look-up-table (3 D LUT) may be applied to the digital color image in order to modify the color or tone scale of the digital color image. All of these methods of tone scale application alter the sharpness of the detail of the modified image, and the latter can produce displeasing artifacts.
Traditional methods of applying a tone scale function to a digital color image modify the apparent sharpness of the image because the tone scale modifies the amplitudes of high frequency detail information. This phenomenon occurs when the tone scale function is applied to each of the color channels independently, or when the tone scale function is applied to the neutral channel and original pixel color difference signals are preserved.
In an effort to apply a tone scale function to a digital image without distorting the detail information, in U.S. Pat. No. 5,012,333 Lee et al. proposed separating the image into a high frequency and a low frequency image by using FIR filters. The tone scale function is then applied to only the low frequency image, and the high frequency image is added back to the tone scaled low frequency image.
Also, in U.S. Pat. No. 5,454,044 Nakajima suggests modifying the image contrast by the formula
Sproc=Sorg+f(Sus).
The low frequency image (Sus) is passed through function f( ) which is a monotonically decreasing function. This signal is added to the original (Sorg) to create the processed image Sproc.
Both of these methods preserve the high frequencies of the image, but this may lead to the creation of unsharp mask type artifacts (overshoot and undershoot) in the neighborhood of large edges (characteristic of occlusion boundaries or dark shadows).
There exist in the prior art methods to modify the color and tone of a digital color image with a 3 D LUT. For example, the modification of a digital image by a 3 D LUT is described in U.S. Pat. No. 4,500,919. However, this method also modifies the appearance of detail in the processed digital image. None of the methods described in the prior art address the need for detail preservation when modifying the color and tonal characteristics of a digital color image with the use of a multidimensional look-up-table.
Thus there exists a need for a method to allow the application of a multidimensional look-up-table to a digital image to adjust the macro contrast of the image and modify the color characteristics of the image, while preserving detail information, without introducing artifacts in the neighborhood of large intensity transitions.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for adjusting the color and tonal characteristics of a digital image, the method comprising: (a) receiving a digital image having two or more digital image channels; (b) separating each digital image channel into two signals; (c) modifying one of the signals with a multidimensional transform for adjusting the color and tonal characteristics of the digital image; and (d) combining the output of (c) with the remaining signal for producing an enhanced digital image.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
The present invention has the following advantages of permitting the application of a multidimensional look-up-table to a digital image to adjust the macro contrast of the image and modify the color characteristics of the image, while preserving detail information, without introducing artifacts in the neighborhood of large intensity transitions.